Does keyword location on page affect ranking?

Introduction

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Most search engine experts recommend placing keywords near the top of web pages. Search engines, they claim, place more importance on text in this area. Are they right? I decided to investigate, and see if it's more effective to have your keyword mentioned in the top third, middle third or bottom third of a page...
Methodology 

I gathered the results of the queries naturally performed last month by myself and three associates using Google. I then fetched the pages and divided the body section into three equal parts for each page. I tallied the results for the first eight rankings on Google and then converted them into a percentage of the total results.
Results & conclusion
The X-axis shows the ranking (from #1 through #8) of the search engine results in the study. The Y-axis shows the percentage of domains that contained the keyword in the top (red line), middle (blue line) and bottom (purple line) thirds of the body section of the page.
It is interesting to note that pages containing the keyword in the top and bottom third of the body section ranked much better on Google. The top section had a normalized correlation of +42 on a scale of -100 to +100. The bottom third also showed a remarkable positive correlation of +46 on the same scale. Having the keyword in the middle third had no significant effect (no correlation whatsoever... neither positive nor negative).

As you can see from the above graph, the results are very conclusive. Mention your keywords near the top and/or bottom of a page for Google ranking.
Limitations
There was no attempt to isolate different keywords. I merely took a random sampling of the queries performed by myself and three associates during the month.
This is merely a correlation study, so it can't be determined whether the leading search engines purposefully entertain this factor or not. The actual factors used may be far distant from the factor we studied.
This article was written by Jon Ricerca. Jon is one of the leading researchers and authors of the Search Engine Ranking Factor (SERF) reports at SearchEngineGeek.com.